1. Field of the Invention
The present invention relates to a turbofan jet engine, and particularly to a turbofan jet engine that is suitable for a propelling device for an aircraft that flies at supersonic airspeeds during a cruise.
2. Description of the Related Art
For a supersonic aircraft, an engine with small frontal area is preferable from the view point of drag reduction because a wave drag comes out only at supersonic speed which is largely dependent on the total frontal area of the aircraft including the frontal area of the engines. On the other hand, for such an engine with small frontal area, the speed of the exhaust jet needs to be relatively high and the bypass ratio low compared to the modern high bypass ratio engines for subsonic aircraft under the condition that the same level of thrust is required. The high speed exhaust jet causes a noise problem around the airport because the noise level generally increases in proportion to the eighth power of the exhaust jet speed.
In order to reduce the jet noise, noise suppression devices have been developed which are added downstream of the jet engine such as the lobe mixer and the mixer ejector. Even though they have some effects on suppressing the noise, they are not easy to be in practical use because they are still too long and too heavy compared to the total engine size and weight if it has a considerable effect on suppressing the noise.
In order to obtain the thrust required for supersonic flight while suppressing the noise generated at takeoff, variable cycle engines have been proposed and examined which have incorporated therein a variable mechanism for changing the bypass ratio. U.S. Pat. No. 3,841,091 proposes a system which is configured such that two turbojets are provided at the front and back of the system respectively, and bypass air is fed to one of the turbojets by switching the air duct.